1)they should, but there is issue with pitlines in Mumbai
2)it doesn't provide benefit to SUR. the move was more to reduce the disapproval for the move from SUR rather than to provide benefit to them. remember that the train, as well as SUR belong to CR, which is head quartered in MH. the whole move had to be done carefully to avoid angering political masters in MH, and moves like these rarely benefit public. as you mentioned, desperate people will anyways book tickets in two legs (GDG-SUR and then SUR-CSMT) so one can't say that those seats remain for SUR public only.
this...
more... is what one can call as lose-lose situation.
still, something is better than nothing :)